MIT-Bates Polarized SourceT
M. Fai khondeh. W. Franklin, E. Tsentalovich, T. Zwart and E. Ihloff
MIT-Bates Linear Accelerator Center
P.O.Box, 846, Middleton, MA 01949, USA
Abstract. During the fall of 2001, the polarized source at MIT delivered over 140 Coulombs of high
quality polarized beams to the SAMPLE-III 125 MeV parity violating experiment. Prior to the
experiment, the source was reconfigured to deliver highly polarized beam using the new high power
diode array laser system with large aperture beam optics, and a strained layer GaAsP photocathode
from St. Petersburg. The results of these tests will be presented. The production run for SAMPLE-III
was then carried out with a bulk GaAs and the Ar-Ti: Sapphire laser system. Since April of this year, the
source has been delivering high polarization beams to the South Hall Ring for commissioning of the
BLAST spectrometer. The stored current in the ring exceeds 100 mA. This is accomplished using the
high power diode laser system and high-gradient-doped strained GaAsP photocathodes from Bandwidth
Semiconductor Inc. tuned for 810 nm. The operational lifetime of the photocathode is excellent. A
status report of the Bates polarized source and the operational experience of delivering high polarization
beam to SHR will be presented.
INTRODUCTION
The MIT-Bates accelerator complex is capable of operation in three distinct modes.
These are the long pulsed mode used for the SAMPLE parity violating experiment [1]
in the North Hall, the storage mode, and the extraction mode using the South Hall
Ring (SHR). A schematic diagram of the MIT-Bates accelerator complex is shown in
Figure 1. The pulsed mode consists of pulses up to 35 |LLS long, 10-mA peak at 600 Hz
providing 40 |nA average currents for the SAMPLE experiment. For the storage mode,
multiple 1.3 |LLs long pulses at low repetition rates are injected into the SHR and
stacked to circulating currents exceeding 80 mA for internal target experiments. The
extraction mode employs 1.3-|LLs long -20 mA pulses from the source at 600 Hz for
injection into and cw extraction from the SHR to a fixed target beam line. The
requirements for injecting high polarization beams for the storage mode are relatively
modest, but very demanding in peak current and repetition rate for the extraction
mode. In this paper the polarized source for the SAMPLE experiments and for the
commissioning of the BLAST with SHR will be described.
POLARIZED BEAMS FOR SAMPLE
The 200 MeV SAMPLE experiments on hydrogen and deuterium [2,3] were
successfully completed in 1998-1999 with beams of unprecedented quality and
Work supported in part by the Department of Energy under a Cooperative agreement # BEFC294ER40818.
CP675, Spin 2002:15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. L Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
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stability using bulk GaAs photocathodes and an Ar-Ti: Sapphire laser. The polarized
injector and the laser trajectories are shown schematically in Figure 2. Prior to the
start of the 125 MeV SAMPLE-III experiment, a period of 1.5 months was dedicated
South Hall Riraj
Figure 1. A schematic view of the MIT-Bates Linear Accelerator Center showing the linac,
recirculator, the South Hall Ring, and the three experimental areas.
to the evaluation of the feasibility of utilizing a newly available high polarization
beam for SAMPLE. The results of these studies will be presented in the next section.
The SAMPLE-III production run was completed using 140 Coulombs from a bulk
GaAs photocathode and the existing Ti:Sapphire laser system.
HIGH POLARIZATION BEAM DEVELOPMENT
To meet the Bates requirements for highly polarized beams, a 60 keV test beam
setup with a Mott polarimeter was completed in 2000 and is now in routine use. This
setup allows research and development with high power diode lasers and high
polarization photocathodes independent of the main accelerator. In fall of 2001, a
multimode fiber-coupled diode array laser system [4] capable of producing peak
power up to 150 W unpolarized radiation at ^=810 ±3 nm was installed on the main
injector. The parameters of this laser system are listed in Table 1. The large 200 mmmrad emittance of this laser precluded use of the existing 20-m transport line for the
Ti:Sapphire laser. A new wide-aperture 4-m long laser transport system matching the
emittance of the diode laser system was designed and installed. The transport system
consists of an optical board installed on a precision six-strut alignment system, 2-inch
aperture focusing lenses and polarizers, and a 70-mm aperture helicity Pockels Cell [5]
(HPC).
TABLE 1. The parameters for the high power
diode array laser systems.
Value
Unit
Wavelength
nm
810±3
Emittance
200
mm.mr
Peak power
150
W
Repetition rate
1-cw
Hz
Pulse-to-pulse jitter
%
<0.1
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(I Injei
Figure 2. A schematic view of the MIT-Bates polarized injector showing the gun, the injector
beam line and the laser paths for both the Ti: Sapphire and the new diode lasers.
The laser beam spot size can be adjusted to illuminate half to full 11-mm diameter of
the photocathode by adjusting the longitudinal location of the last converging lens
near the end of the transport line. The trajectory of this laser is also shown in Figure 2.
The laser system and the associated hardware are installed at ground potential outside
the Faraday cage and the acceleration column both at 300 keV potential. The laser
beam enters the gun chamber directly through a vacuum port and strikes the
photocathode at an incident angle of 37° with respect to the normal. Because the index
of refraction of GaAs based material is 4.5, the refracted ray in the GaAs is close to
the normal (0r ~ 8°), and there is no significant reduction in the electron beam
polarization. However, the -37° incident angle has influence on the helicity-correlated
effects due to differences between the reflectivities of the residual linear polarization
of the S and P waves.
High Polarization Beam Tests for SAMPLE-III
A gun was prepared with an As-capped strained GaAs photocathode from the St.
Petersburg group [6] and installed for high polarization beam tests for SAMPLE-III
experiment. Measurement with the SAMPLE Moller polarimeter at 125 MeV yielded
a beam polarization of only -55% at the diode wavelength of 810 nm. It was then
discovered that this sample was not optimized for our laser and was designed to
produce maximum polarization of -75% at 850 nm. However, the lifetime and the
pulse-to-pulse stability were excellent and exceeded the SAMPLE requirements. The
experiment placed stringent limits on helicity-correlated beam position (<100 nm) and
intensity (< 10 ppm) differences at the SAMPLE target (averaged over 1/2 hour). Due
to birefringing effects in the optical elements and the vacuum window, an asymmetry
is generated in the transport of the laser light for the two helicities due to residual
linear polarization in the light. In addition, strained photocathodes are known to have a
preferred optical axis causing an analyzing power of the order 5-10 %. The non-zero
incident angle of laser adds to these asymmetries. These analyzing powers create
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helicity correlated asymmetries in the position and intensity of the beam. A halfwave
plate was inserted on the laser table between the HPC and the photocathode and
rotated to minimize these differences. A piezo-electric system [7] was also used for
slow feedback to reduce the position differences. More details on these tests are found
in the presentation of E. Tsentalovich in this workshop [8].
High Polarization Beam for SHR
A gun with a high-gradient-doped strained GaAsP photocathode grown by
Bandwidth Semiconductor Inc. [9] with the SLAC specification [10] was prepared and
installed on the injector in 2002. With 5% phosphate concentration, the peak of
polarization is lowered from 850 nm to -810 nm matching the wavelength of our high
power diode laser. The top 10 nm layer of this sample is GaAs and is heavily Zinc
doped (5xl019) to minimize the surface charge limit effect often present in strained
samples at high laser power densities. However, diffusion of Zinc during heat cleaning
at -600 °C will cause a depletion of the dopant concentration in the top layer. This
will lead to a gradual appearance of surface charge limit after several one-hour long
heat cleanings. The degradation is shown in Figure 4 for the first high gradient doped
photocathode that was in use on the main injector for a five months period between
April and September of this year.
10 nm 5x1019cni*
9Qnm 5x1017em"$
2.5 jim
M"0 -> 0.34
GaAs substrate
Figure 3. A schematic diagram of a high-gradient-doped strained GaAsP photocathode grown by
Bandwidth Semiconductor Inc. [9] with the SLAC specification [10].
This type of photocathode and the high power diode array laser system [11] have been
in routine use injecting beams with polarizations exceeding -70% for the BLAST
commissioning. The second high-gradient-doped strained photocathode has been in
use since September and has displayed a superior performance compared to the first
sample. One reason is that for this second sample, we are limiting the heat cleaning
duration and temperature to -10 minutes and -575 °C respectively. This should reduce
the delusion rate of the Zinc dopant out of the heavily doped layer, thus reducing the
magnitude of the charge limit effect. The beam polarization in the SHR is measured
using a laser back-scattering Compton polarimeter [12] in the last arc upstream of the
internal target and BLAST area. This polarimeter provides noninvasive continuous
monitoring of the beam polarization in the ring with roughly 5% statistical accuracy
in a half-hour run. The Compton polarimeter and the SHR form a good combination
for studying the spin dynamics in storage rings. A transmission polarimeter [13] is
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periodically used at the 20 MeV region of the linac for fast relative measurements of
the beam polarization near the injector.
9/26/02
Figure 4. A graph of peak current in mA vs. calendar time for the 5 months period that the first high
high-gradient-doped strained GaAsP sample from Bandwidth Semiconductor Inc. [9] was in use for the
BLAST commissioning. To show the degradation of the QE over this period, the peak current is plotted
for a fixed laser power of 18 W. The location of each activation is shown by a vertical arrow.
CONCLUSIONS
Three SAMPLE experiments were successfully completed using over 450
Coulombs of polarized beams of high quality originated from unstrained bulk GaAs
photocathodes. A test of high polarization beams using strained GaAsP and a high
power diode laser system showed photocathode lifetimes and intensity stabilities
exceeding SAMPLE-III requirement, but helicity correlated beam effects require
further development. The polarized injector is now in continuous operation, injecting
highly polarized beams into the SHR for the BLAST commissioning runs.
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